An X-ray spectral variability of fast disk winds in AGN
Kouichi Hagino,1 Hirokazu Odaka,2 Chris Done,1,3 Shin Watanabe,1,4 Tadayuki Takahashi1,41Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA),
3-1-1 Yoshinodai, Chuo, Sagamihara, Kanagawa 252-5210, Japan
2KIPAC, Stanford University, 452 Lomita Mall, Stanford, CA 94305, USA
3
Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK
4
Department of Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
E-mail(KH): hagino@astro.isas.jaxa.jp
Abstract

Recent X-ray observations of blue-shifted absorption lines revealed an existence of the extremely fastdisk winds with outflow velocities of ∼ 0.1–0.3c. Such fast outflows would have a large impact on thecoevolution of black holes and host galaxies since they are expected to carry a large amount of kinetic
energy. One of the common characteristics of these fast winds is a strong time variability of the absorption
feature. To investigate this variability, we have developed a new X-ray spectral model of the disk winds,
which is generated by 3-dimensional Monte Carlo radiation transfer simulations on the assumption of therealistic wind geometry. By applying our wind model to the multi-epoch X-ray data of an archetypal
wind source PDS 456, we find the variability in the absorption line is explained by a change of thewind outflowing angle without any large variability in the mass outflow rate of the wind. This result
indicates that the fast disk winds are stable and that local hydrodynamic instabilities produce a large
time variability of the absorption line. Moreover, we also apply our wind model to the disk-line source
1H 0707−495. The characteristic Fe-K feature in this source is successfully reproduced our disk windmodel for all the observations.
Key words: black hole physics — radiative transfer — galaxies: individual (PDS 456, 1H 0707−495)

Introduction!"!%#&!"!%

Broad iron line Blueshiftedfrom the wind absorption
lines
Continuum
absorption$&!"!'()*#+,-./0/12+34!#+2!!+()*!!5

PDS 456: 2007-02-24
PDS 456: 2013-03-08#&!"

Blueshifted absorption lines with velocities of 0.1–0.3c
from highly ionized iron ions have been discovered inmany local active galactic nuclei (AGN) by recent Xray observations (e.g., Chartas et al. 2002; Reeves et al.
2003; Pounds et al. 2003a; Pounds et al. 2003b). Theseabsorption lines indicate that heavy ions are outflowing with such extremely fast velocities from the vicinity of supermassive black holes at the center of galaxies.
These outflows are often referred to as ultra-fast outflows(UFOs) (Tombesi et al. 2010). Since they are expected tocarry large amounts of kinetic energy to the host galaxies, they are thought to play an important role in theco-evolution of the black holes and the galaxies (King &Pounds 2015).
In spite of its importance, physical properties of UFOs
are still unclear. Particularly, the origin of spectral
variation of the blueshifted absorption lines in UFOs
is unknown. As an example, spectra of PDS 456 observed in 2007 and 2013 by Suzaku are shown in Fig. 1.
These spectra clearly show spectral changes in both absorption lines and continuum absorption, which are a!'1.

#$61)789+,()*5!"

Fig. 1. Observed spectra of an archetypal ultra-fast outflow inPDS 456.

10.4
0.3
0.2

Total
DirectReprocess

0.1
0.04
0.03
0.02
0.01

Normalized ﬂux

Normalized ﬂux

45­47 deg257­59 deg

210.4
0.3
0.2

Total
Direct0.1

Reprocess

0.04
0.03
0.02
6810
Energy (keV)

12140.01

6810
Energy (keV)1214Fig. 2. Simulated spectra with diﬀerent angle (Hagino et al. 2016).

common interesting characteristic in UFOs. Since theblueshifted absorption lines are usually interpreted as
K-shell transitions of He-like and/or H-like iron ions,outflowing materials should be highly ionized with anionization parameter of ξ ≡ L/nR2 ∼ 103–4 . On theother hand, such highly ionized material is not able toexplain the strong continuum absorption as shown in
Fig. 1, which requires relatively low ionized materials
with ξ < 102 . Thus, these materials are often thought∼
to be cool clumps embedded in the hot highly ionizedoutflow. These clumps are naturally expected in hydrodynamic simulations of the accretion disk winds due
to ionization instability and/or hydrodynamic instability (Krolik, McKee & Tarter 1981; Takeuchi, Ohsuga,
Mineshige 2014). While occultation by the cool clumps
can easily explain the spectral variability in continuum
absorption (Matzeu et al. 2016), there is no clear picture for explaining the spectral variation in blueshifted
absorption lines produced by the highly ionized outflow.
2. Monte Carlo radiation transfer simulations
We construct a new spectral model of the UFO in orderto understand the spectral variability of the highly ionized absorption lines (Hagino et al. 2015). In this model,
we assume the UV-line driving mechanism. This is thebest candidate of acceleration mechanism of UFOs because of its good eﬃciency in AGN environment. In theUV-line driving mechanism, moderately ionized materials are accelerated by radiation pressure due to boundbound transitions with UV photons. Since cross sectionsof bound-bound transitions are 3–4 orders of magnitudelarger than that of Thomson scattering, this mechanism
can eﬃciently accelerate and launch the fast disk wind
if materials are appropriately ionized. Moreover, since
AGN radiation typically peaks at the UV band, which

moderately ionizes materials and causes radiation pressure via bound-bound transitions, AGN is suitable for
the UV-line driving.
We assume a biconical geometry, which was developed
for accretion disk winds in cataclysmic variables (Knigge,Woods & Drew 1995). Based on the hydrodynamic simulations of the UV-line driven disk winds, a covering fraction of Ω/4π = 0.15 is adopted (Proga, Stone & Kallman
2000; Nomura et al. 2016). A maximum velocity of thedisk wind is determined by the observed absorption lines,and its distribution in the disk wind is calculated following the CAK velocity law (Castor, Abbott & Klein 1975).
A density distribution is derived from the velocity distribution and conservation of mass. Then, an ionization
structure of the wind is calculated 1-dimensionally along
stream lines by utilizing XSTAR (Kallman & Bautista2001), which outputs ion abundances and electron temperatures in the wind. The radiation transfer calculationis performed in these density, velocity, ionization distributions.
We adopt Monte Carlo method for the radiation transfer calculation in the UV-line driven disk wind because its non-spherical geometry makes it very diﬃcult
to solve with analytical methods. The Monte Carloradiation transfer simulation is performed by utilizing
a simulation framework called MONACO (Odaka etal. 2011). MONACO is a general-purpose framework,
which have been applied for synthesizing X-ray spectrafrom many kinds of astrophysical objects (e.g., Odakaet al. 2014; Odaka et al. 2016; Furui et al. 2016; Hagino
et al. 2015; Hagino et al. 2016). In this code, all important physical processes in photoionized plasma are
implemented, namely photoionization, recombination,photoexcitation, Compton scattering (Watanabe et al.
2006). In addition to these, special relativistic eﬀects,
which are essential in UFOs, are also considered.

fast, thinslow, denseθ
X-raysource

θ=46 deg
θ=58 deg

accretion disk

Fig. 3. A schematic view of angle dependences of the absorption
lines.

X-ray spectra calculated by our simulations are shownin Fig. 2. As shown in the left panel, blueshifted absorption lines and a broad emission line similar to theobservations are reproduced. More interestingly, when
the wind is observed from a larger viewing angle, theabsorption lines become deeper and broader as shown in
the right panel. Fig. 3 schematically show how the absorption line features depend on the observer’s viewing
angle. At a smaller viewing angle, only a thin and fastpart of the wind is observed. On the other hand, at alarger angle, a slow and dense part is observed as well as
the fast and thin part, and it makes the absorption lines
broader and deeper.
3. Application to an archetypal wind source PDS 456
We apply our spectral model to observational data ofan archetypal disk wind source PDS 456. This source isobserved by Suzaku for 5 times between 2007 and 2013,and most of these data clearly show blueshifted absorption lines (Reeves et al. 2009; Goﬀord et al. 2014). Also,
as already shown in Fig. 1, the absorption line featuresstrongly vary. It means that this source is one of thebest target to study the origin of the spectral variationin the highly ionized absorption lines.
All the Suzaku spectra of PDS 456 are successfully
reproduced by our wind model with a mass outflow
rate of Ṁwind /ṀEdd = 0.13 and a terminal velocity ofvwind ∼ 0.3c. As examples, we show the spectra andbest-fit models for observations in 2007-02-24 and 201303-08 in Fig. 4. Between the observations, we change
only the viewing angle and the terminal velocity withoutchanging the global parameters such as the mass outflow
rate.
A change of the viewing angle, which successfully explains the strong spectral variability of absorption lines,
possibly relate to a flapping or/and an inhomogeneity ofthe disk winds. They are naturally expected in hydrodynamic simulations of the winds due to hydrodynamic
instability (Proga, Stone & Kallman 2000; Krolik, McKee & Tarter 1981; Takeuchi, Ohsuga, Mineshige 2014).
Thus, our result indicates that the strong variability of
the highly ionized absorption lines in UFOs could originate from a local instability of the wind, not from aglobal change of the wind.
4. Application to a disk-line source 1H 0707−495
As a next target, we choose a narrow line Seyfert 1 galaxy
1H 0707−495, which is famous for its extremely smeared
disk-line feature (Fabian et al. 2004). However, this diskline interpretation for a characteristic iron-K spectralfeature in this source requires extreme conditions. Since
the iron-K line is extremely smeared, a black hole spinmust be close to maximum. Moreover, an X-ray emitting
region (corona) must be located very close to the event
horizon to explain its strong reflection component. Also,a large iron abundance of 7–20 is required by its strong
iron line.
Alternatively, we interpret the iron-K feature as anabsorption feature produced by the UFO. This interpretation is very natural because the spectral features in
1H 0707−495 is very similar to those in PDS 456. Also,
this interpretation is supported by optical data of thissource. It requires super Eddington accretion (Done &Jin 2016), and in such a situation, the accretion disk
must not be a standard thin disk, which is assumed in
the disk-line interpretation.
We apply our disk wind model to all 15 spectra of
1H 0707−495 observed by XMM-Newton and Suzaku,
and successfully reproduce the structure at iron-K bandby changing only the viewing angle (Fig. 5). It meansthat the spectra of this source can be explained by theUFO with Ṁwind /ṀEdd = 0.2 and vwind = 0.2c. In addition to these data, we also compare an extrapolation of
the best-fit wind model for Obs15 (right panel of Fig. 5)
with the NuSTAR data (Kara et al. 2015). We find thatthe higher energy data is also explained by our disk windmodel. This strongly supports our disk wind interpretation for this source.
5. Conclusions
We have constructed a new spectral model by calculating the radiation transfer in a realistic wind geometry
based on the UV-line driving mechanism. We first applied our disk wind model to an archetypal wind source
PDS 456, and found that the strong spectral variation inthis source can be explained by a change of the viewing
angle without changing a mass outflow rate. It indicatesthat the spectral variability is due to a local instability
or inhomogeneity of the wind. As the next target, weapplied to the “disk-line” source 1H 0707−495. Strong

()*%+,-./0/12+34!%+2!!+()*!!5()*%+,-./0/12+34!%+2!!+()*!!5%&!"!$!"!$Δθ=3°
χ2ν=127.9/107'&!"!#%&!"!#

2007!"!#%%&!"!$!"!$Δθ=10°
χ2ν=88.5/89'&!"!#%&!"!#

2013c!"!#%!

!

!

""
!!

!%
'61)789+,()*5!%!"'61)789+,()*5!"!"!%&'()*+,-./.01*23!)*1!!*&'(!!4&'()*+,-./.01*23!)*1!!*&'(!!4Fig. 4. Suzaku spectra of PDS 456 compared with our spectral model. ∆θ indicates a diﬀerence between the observer’s line of sight and an
inner stream line of the wind (Hagino et al. 2015).

561!)

Δθ=19.1°
χ2ν=52.5/55!"!$!"!#)!"!%

561!#Δθ=25.9°
χ2ν=43.1/51!"!$!"!#)
!

!

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790':;<*+&'(48!")$

790':;<*+&'(48!"Fig. 5. XMM-Newton spectra of 1H 0707−495 compared with our spectral model. ∆θ indicates a diﬀerence between our line of sight andthe inner stream line of the wind (Hagino et al. 2016).

Fe-K features in all the spectra of this source observed
by XMM-Newton/Suzaku are successfully reproduced byour disk wind model. Moreover, higher energy spectra
obtained by NuSTAR are also explained by our disk windmodel.
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